Should transformer core be grounded?

[spartanv12]

We have numerous 1500kva dry type transformers being installed, they do not appear to have the laminations grounded. The steel beam supporting the laminations is grounded but insulated from the laminations.
We've not tested to see if they are grounded but visually they don't appear to be.

 

[RalphChristie]

Any conducting part of a trsf, unless solidly bonded to earth, will acquire a potential in operation. In theory you can insulate it from earth, but in practice it is easier and more convenient to bond them to earth. By bonding there are two important requirements:

1.Bonding must ensure good electrical contact and remain secure throughout the transformer life. Metalwork which becomes inadequately bonded, creates arcing which will cause breakdown of insulation and oil.
2.No conducting loops must be formed, otherwise circulating currents will result.

 

[electricpete]

Yes - should be grounded at one point only.

I did see a G.S.U. transformer manufactured by Schmitt (Netherlands) where the core was not "solidly grounded", but instead had resistors in the core ground circuit for some reason. Maybe to limit circulating current in the event of a 2nd ground.

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[ScottyUK]

epete,

Believe company name is 'Smit Transformatoren BV'

http://www.smittransformers.com/

Interesting idea about resisitve core grounding - I'll have to remember that one.





----------------------------------

One day my ship will come in.
But with my luck, I'll be at the airport!

 

[stevenal]

IEEE C57.12.01 requires the core to be grounded to the frame and enclosure.

 

[Luis1947]

There is a protection named "Tank earth leakage", which sense the current to earth through a current transformer connected between the tank and ground. Overcurrent relay is necessary also. In this case it could be between the core and ground.

As an example see

http://www.schneider-electric.com.t...ection/sepam2000_transformer_catalogue_en.pdf

See page 7 for description and page 17 for wiring connections.
Other suppliers have similar devices

Maybe your transformer has similar protection

Regards
Luis

 

[spartanv12]

Thanks guys,

The transformers are manufactured in China and badged ABB,
they are installed with no protection other than the electronic WTI Hi temp shunt trip and fuses at the 11kv RMU.
I'm fully conversant with tank earth fault protection our 10mva mobile skid transformers have this.

I'll endeavour to read the standards and also give the core an insulation test to ground.

I have had little experience with these transformers until recent.
Reading info on the web it appears that there are expoxy resin type and another type.
Unsure as to what type we have, but from China they are probably cheap!

 

[fsmyth]

FWIW, small transformer cores are almost always
grounded (at least, not insulated from the chassis).
I would be very interested to learn about any
potential difference between the core and ground,
if there is any in a healthy transformer. I have
measured a small capacitively coupled voltage on
transformers with electrostatic shielding between
windings, but not often. I would think that there
would be NO measureable current in a loop or T core
unless something was wrong (cracks, separations, etc.)
It would make sense to ground the core as a safety
measure, IMHO. Are pole pig cores grounded?
<als>
P.S. I should add that my experiences are all on
10 Kva or less. Is the big transformer world that
much different?

 

[skiier]

Has anyone forgotten "transformers 101." Correct me if I am wrong but isn't the purpose of insulating the laminations from one another to REDUCE the effect of EDDY currents and their damaging heating effects?
"The steel beam supporting the laminations is grounded but insulated from the laminations." No duh. This is so the supporting beam does not SHORT OUT the laminations.
All you gnd happy people should take in the PRACTICAL considerations of why the core is NOT gnded. It's not gnded so that it won't produce eddy currents and overheat and burn up. PERIOD.
Oh yes the shock hazard you talk of is GREATLY reduced if one encloses the core in a METAL CASE to keep idiots from touching the ELECTROSTATICALLY CHARGED core and killing themselves. Did I say authorized personnel only? Is there a sticker n the enclosure?
Oh yes, if you went and looked at a TF you would see a little sticker on it that says those nasty metal punched open sided killer T/F's are NOT be installed anywhere accessible to the public. And don't mess with the design. I think it says that too.
Sorry to be abrupt. Let's just be practical for once.

skiier

 

[electricpete]

skieer - abrupt but wrong. Most transformer cores are grounded. A single point ground does not create any path for circulating current. Even if the laminations are bridged together at one point, there is no path for circulating current. Current requires a loop to flow.

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[electricpete]

It's good skieer that you point out that if we had multiple supports with none of them insulated we would have multiple shorts across laminations and circulating current. Maybe that answers the original poster's question of why the insulation was there.

As stated above, you are wrong to suggest that transformer cores are left floating.

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[edison123]

Though I never understood it, all the oil filled power trafos that I have handled have had a ground connection through a thin strip of copper that was located in the centre of top yoke laminations.